Various insulating materials to be used as coatings have been developed to strengthen the resistance of underlying substrates to increased temperatures. Thermal Barrier Coatings (TBCs) are commonly used to protect a machine's critical components from premature breakdown due to increased temperatures to which the components are exposed. Generally, TBCs extend the life of critical components by reducing the rate of metal waste (through spalling) by oxidation.
A fundamental drawback of TBCs is a limitation in thickness that can be used. This thickness limitation of approximately 0.5 mm is due to manufacturing-induced residual stresses, prohibitive costs, required life of the TBC material, temperature limit of the TBC, and mismatches in the coefficients of thermal expansion of the TBC and the substrate. In addition, microstructure of conventional TBCs (those applied by both air plasma spray and physical vapor deposition) is dictated by process conditions, is limited in versatility, and is prone to dimensional and thermal instability at temperatures greater than 1000.degree. C.
Materials comparable to TBCs are fibrous ceramic insulating materials. A major drawback of these materials, however, is that they have low densities which lead to very poor erosion resistance. Therefore, fibrous ceramic insulating materials are inapplicable to high velocity gas flow applications.
Monolithic tiles are another material to be used for protecting critical components in high temperature conditions. These tiles have good erosion resistance and insulating properties, however, they are susceptible to thermal shock damage and catastrophic failure. It is, therefore, desirable to provide insulating materials that can withstand high temperatures without the use of thermal barrier coatings, fibrous ceramic insulating materials, or monolithic ceramic tiles.
Commercially available ceramic matrix composites (CMCs) have many potential applications in high temperature environments. CMCs, however, are limited in their exposure to temperatures near 1200.degree. C. for long periods of time. In addition, CMCs cannot be effectively cooled under high temperatures (&gt;1400.degree. C.) or high heat flux conditions because of their lower conductivity than metals and their limitations in cooling arrangements due to manufacturing constraints. It is, therefore, desirable to provide a material that can be used to insulate moderate temperature ceramic matrix composites, is also erosion resistant, thermal shock resistant, and has coefficients of thermal expansion relatively similar to that of CMCs.
European Patent Office publication No. 007,511,04, entitled "An Abradable Composition," filed Jan. 2, 1997, discloses a ceramic abradable material that can be used to seal ceramic turbine components. This material, however, purportedly has a high temperature capability of only 1300.degree. C.